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All types of air compressors share an ambient temperature sensitivityxe2x80x94both the capacity and the efficiency decrease as the ambient temperature increases. The compressor-specific power demand is approximately proportional to the absolute temperature, which makes the efficiency proportional to the inverse absolute temperature. The compressor capacity is proportional to the density of the inlet air.
These sensitivities become particularly pronounced in combustion engines, in which the compressed air is used to combust a fuel and ultimately produce power. Both the power output and engine efficiency are de-rated at warm ambients. The degradation is not so severe with reciprocating engines, which require little more than stoichiometric air. The degradation is very severe with combustion turbines, which require on the order of 3 or 4 times stoichiometric air.
One known method of counteracting the warm ambient degradation of air compressors is by cooling the inlet air, either evaporatively or with a refrigerant. The refrigerated cooling can be done either in refrigerated air coils or by direct contact with sprayed chilled water. The refrigeration is supplied by either mechanical or absorption refrigeration systems, and in some instances through a cold storage medium (ice or chilled water).
Another approach to cooling inlet air is by over-spraying, typically via fogging. Sufficient water is injected into the air in fine droplet form such that it not only reduces the temperature adiabatically to the dew point, but additional droplets remain un-evaporated, and carry into the compressor suction. Those droplets rapidly evaporate as compression proceeds, slowing the temperature Increase caused by compression, and hence effectively adding to the amount of inlet cooling. For the droplets to remain suspended in the air into the suction rather than separate out excessively, they should be in the fog-size range, i. e., less than 40 microns in diameter and preferably 5 to 20 microns, Another advantage of this size range is that the droplets are small enough that they do not erode the compressor blades.
The problems with the current approaches to cooling compressor inlet air include the following. Most compressors would benefit thermodynamically from sub-freezing inlet temperatures, or at least could be designed to benefit from those temperatures. However, there are many practical difficulties. Especially with high rotational speed combustion turbines, there is a possibility of ice buildup on inlet guide vanes, which then could spall off and damage the compressor blades. This imposes a practical limiting temperature of about 4xc2x0 C. for many inlet cooling systems. Cooling below that temperature will require some additional technique of reducing the humidity level of the cold air below saturationxe2x80x94reheat, etc. On the refrigeration side, special measures are also required to deal with the H2O removal from the air in sub-freezing conditions: periodic defrosting of the air coils, or continuous addition of a melting agent. Furthermore, the refrigeration system requires proportionately more input power to reach the lower temperaturesxe2x80x94more shaft power for mechanical refrigeration, or higher quality heat for absorption refrigeration. With mechanical refrigeration, the power necessary to reach sub-freezing temperatures is so large, and the marginal improvement in compression is so small, that there is little or no net gain from cooling to sub-freezing temperatures.
Even when the inlet cooling is restricted to above-freezing temperatures, another major problem remains. The compressor benefit is substantially due to the sensible cooling of the inlet air, with almost no added benefit from the latent cooling, i.e., the amount of moisture condensed out of the air. However, the latent cooling typically represents 25 to 50% of the total refrigeration load. For example, consider 35xc2x0 C. air at 50% relative humidity, which is cooled to 5xc2x0 C. at 100% relative humidity. The moisture content decreased from 1.8 weight percent to 0.55 weight percent. For these conditions, only 51% of the total refrigeration provides sensible cooling, and 49% causes the water condensation. Thus, much of the refrigeration is effectively wasted.
The overspray or fogging approach to inlet cooling also presents problems. The two foremost are that the cooling is adiabatic, as opposed to the diabatic cooling of the refrigeration approach; and that a source of pure water is required for every bit of cooling accomplished. The adiabatic limitation causes the inlet sensible temperature to be no lower than the dew point. The cost and availability of pure water mitigate against this approach at many sites.
What is needed, and included among the objects of this invention, are apparatus and process which overcome the prior art problems cited above, i. e., an inlet cooling system wherein the latent load contributes to effective cooling in addition to the sensible load contribution; where the benefits of the overspray approach are available without the limitations of needing a large source of pure water and that the inlet temperature is limited to the dew point; where the thermodynamic benefits of sub-freezing inlet temperatures are achievable without the practical problems; and wherein the refrigeration system is activated by low temperature waste heat so as not to detract from the compressor shaft power reduction provided by the inlet cooling system.
The above advantages are obtained in a process for compressing air comprising: chilling air to between the dew point and the frost point; collecting the resulting condensate; injecting the condensate into the chilled air in the form of very small droplets; and compressing the chilled droplet laden air. They are also obtained in an apparatus for increasing the capacity and efficiency of an air compressor comprising: a means for air chilling which is supplied with a refrigerant; a condensate collection system for condensate condensed from said air by said means for chilling; a means for converting said condensate into fog-sized droplets; a means for injecting said droplets into said air downstream of said chilling means; and a duct for supplying said chilled and fogged air to the suction of said air compressor.